Rachel and Chandler ask for six volunteers to participate in human trials of her vaccine, but the tests could be a living hell as the virus and the vaccine do battle inside their bodies. Meanwhile back in the States, Chandler’s wife and father are doing everything they can to keep their family alive.

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EDITOR'S NOTE: The original summary, which I have edited to suit this wiki, was originally written on IMDB.

Out in the woods of Virginia, a man brings a deer he bagged to a trailer, but the people inside warn him they're sick and tell him to stay away. He announces he won't let them starve, then leaves the deer, marking the trailer with a red X.

Back at his cabin, Tom Chandler's father Jed announces to Darien Chandler and three kids that the hunting was scarce. She and the kids did well scavenging in nearby Plainview. She saw an electronic store and wants to go back for the radio part Mr. Chandler needs to fix his radio to reach Tom. He cautions her to bring a weapon.

On the USS Nathan James, Chandler and CMC Hugh Jeter stand over the body of Lt. Cossetti. Chandler feels guilty that Cossetti gave his life for redemption, but Jeter reminds Chandler that Cossetti gave his life for the mission.

In the lab, crewmen help Dr. Rachel Scott rearrange and prepare for human trials of her vaccine. She's going to inject six people of differing genetic make-up with the vaccine and then with the virus, but Tex thinks she'll be short on volunteers.

In comms, Mason lets Bertrise listen to a radio transmission from a family member, but it just makes her sad. She's resolved to be there for the vaccine trials since it was made from her blood.

After Cossetti's formal funeral with gun salute, his body is laid to rest at sea. Chandler, XO Mike Slattery and Jeter discuss how they'll get six volunteers for the trials. Chandler says someone in leadership has to volunteer, and suggests himself, but Jeter says he's already been approved by Dr. Scott to participate.

"I told you god put me here for a reason, maybe this is it," Jeter says.

Chandler pays a visit to the Tophets. Quincy thanks Chandler for giving him a second chance.

Doc Rios and Quincy interview the dozens of volunteers. Dr. Scott tells Danny Green that given the fact he just survived dengue fever, he can't volunteer. But they accept his former girlfriend Lt. Kara Foster, along with Tex, Chief Engineer Andrea Garnett, Miller and one other crew member, Maya.

Dr. Scott injects them with the vaccine, warning them they'll likely feel some of the early symptoms of the disease. Then she ushers them into the quarantine room where they'll remain for the next three days and injects them with the virus.

In the electronics store, Mrs. Chandler looks for the part she needs for the radio when she hears someone shouting outside. She looks out and sees a man (Titus Welliver) with a gun warning an infected woman to stay back. When the woman doesn't stop, he shoots her as Mrs. Chandler hides -- right near an infected dead body.

Dr. Scott checks on the volunteers and Tex continues his flirting with Dr. Scott. Maya goes through old emails from her boyfriend, telling Miller that she's an only child and her parents are dead, so he might be the only one missing her.

Standing outside the quarantine with Slattery, Tom remembers when his son Sam was born prematurely and all he could do was wait in the NICU.

Dr. Scott sees a photo of Andrea Garnett's daughter and tells her she hopes she could be back with her within a week.

Lt. Alisha Granderson gets an update six hours into the trial and the crew gets excited about the idea they could go home soon with a vaccine. Danny Green shuts them up, telling them not to jinx it.

In the quarantine, Kara Foster is running a fever but Doc Rios assures her it's just her body fighting the sickness.

Miller notices his throat is sore. Jeter and Tex seem fine.

Waiting outside the quarantine, Slattery tells Chandler he was right to have faith in Dr. Scott.

In the quarantine, Tex is talking to Kara when she passes out and starts seizing. Dr. Scott, Rios and Tophet tend to her but she's not responding. Bertrise brings a bucket of ice into the quarantine area when Dr. Scott calls for it. They're able to get her fever down.

Later, Tophet worries they didn't build a strong enough virus decoy from Bertrise's receptors, but Dr. Scott thinks they checked it a dozen times and it has to be something in Kara's medical history. She tells Quincy to give the others something to help with the symptoms.

Lt. Carlton Burk and Danny Green play cards, but Danny is completely distracted worrying about Kara.

In quarantine, Maya tells Miller that he parents died during 9/11 when her firefighter father went into the north tower to try to save her mother. That's why she joined the Navy.

Everyone seems to be doing much worse. Garnett has lost blood flow to her fingers and Tex's back is covered in a rash.

The rest of the crew tries to stay busy on the ship.

In quarantine, Garnett is delirious and mistakes Kara for her daughter. Tex is immobilized with the shakes. Dr. Scott sees that Tex has a picture of a young woman in a locket. He grabs her hand and tells her he makes her want to love again.

Jeter stands upright, and starts walking, saying he's ready to see his daughter. Dr. Scott tries to restrain him while Tophet injects him with a sedative.

Miller turns and sees that Maya is still, with blood running out of her nose.

In the cabin, Mr. Chandler works on his radio and they hear a broadcast telling anyone who's sick to come to Olympia.

In quarantine, they wrap Maya's body in a body bag. Chandler wants her given the Navy Cross.

Quincy tries to convince the captain that the vaccine isn't working, but Rachel insists that the heart attack Maya died from isn't a symptom of the virus. Tophet wants to take the plasma from Bertrise's blood and inject them directly with it, but Dr. Scott thinks it'd be too risky for Bertrise. She volunteers anyway.

Later, Dr. Scott draws her plasma and Quincy hands her a file.

Chandler goes to talk to Danny Green and tells him that Kara is pregnant.

They let Danny visit Kara, who now has sores on her head and is bleeding from her nose. The captain says anyone who visits can.

Quincy finds Dr. Scott huddled in a corner, trying to hold it together. The plasma injections aren't making a difference, but Bertrise is going to be OK. Quincy assures her they'll find another way.

In the hallway outside quarantine, Chandler and Slattery are second guessing themselves and are upset they let the trials go forward.

The volunteers are all weak and seem near death. Chandler visits and tells them that when they talk about the great plague of the 21st Century, they'll talk about the six. Quincy is preparing morphine when Dr. Scott comes in with an epiphany.

After realising Bertrice's childhood illness was likely a result from her immune system targeting a self-molecule and leading to an autoimmune response, Dr. Scott explains that what they're seeing is the volunteer's bodies attacking themselves, because after binding to the vaccine, the virus changes its shape to expose a human molecule from Niels' DNA (which is presumably common and essential in normal human body). Then the volunteer's immune system recognises this viral-exposed human component as foreign and attack it along with the same human molecules in the volunteer's body, thus causing the autoimmune responses. It didn't affect the monkey because, of course, he doesn't have human DNA (i.e. his immune system could attack this human molecule freely without the risk of targeting any self-molecules). She thinks if the primordial strain of the virus could be used as a vector to help the body produce a modified version of the vaccine to hide this human molecule, they can stop the volunteer's bodies from attacking themselves. The new drug can hide this human molecule (which also enhances the severity of the original strain by Niels) to prevent the autoimmune responses and weaken the virus for the immune system (now strengthened by the original vaccine) to clean up, thus acting as a cure for the infected.

She gets to work. She whips up some magic potion and injects the volunteers.

In the morning, Mason sits by Bertrise's bedside. The five remaining volunteers are all recovering. Dr. Scott tells Danny Green that Kara's baby will be born immune. He goes to tell Kara the news.

Chandler goes to see Dr. Scott, stunned that they actually have a vaccine. "We don't just have the vaccine, we have a cure -- we can save people who are already sick," she says. He hugs her.

In the Virginia cabin, Jed Chandler gets his radio working and sends out a broadcast looking for the Nathan James. Mrs. Chandler looks queasy (and she just served her whole family water and rubbed her son's face).